Exhaust emissions regulations for internal combustion engines have become more stringent over recent years. For example, the regulated emissions of NOx (also written as NOx) and particulates from diesel-powered internal combustion engines are low enough that, in many cases, the emissions levels cannot be met with improved combustion technologies. Therefore, the use of exhaust aftertreatment systems on engines to reduce harmful exhaust emissions is increasing. Typical exhaust aftertreatment systems include any of various components configured to reduce the level of harmful exhaust emissions present in the exhaust gas. For example, some exhaust aftertreatment systems for diesel-powered internal combustion engines include various components, such as a diesel oxidation catalyst (DOC), a particulate matter filter or diesel particulate filter (DPF), and a NOx adsorber catalyst (NAC) or NOx trap. In some exhaust aftertreatment systems, exhaust gas first passes through the DOC, then passes through the DPF, and subsequently passes through the NAC.
Each of the DOC, DPF, and NAC components is configured to perform a particular exhaust emissions treatment operation on the exhaust gas passing through or over the components. The DOC, DPF, and NAC each includes a catalyst bed or substrate that facilitates the corresponding exhaust emissions treatment operation. Generally, the catalyst bed of the DOC reduces the amount of carbon monoxide and hydrocarbons present in the exhaust gas via oxidation techniques. The substrate of the DPF filters harmful diesel particulate matter and soot present in the exhaust gas. Finally, the catalyst bed of the NAC reduces the amount of NOx present in the exhaust gas.
Generally, the catalyst bed of the NAC is configured to intermittently trap or adsorb NOx and oxygen, and then release or desorb the trapped NOx and oxygen while reducing the released NOx to N2 and other compounds to meet emissions standards. NOx and oxygen is adsorbed on the catalyst bed while the engine runs lean (resulting in excess oxygen in the exhaust gas passing through the NAC). The release and reduction of NOx trapped on the NAC, otherwise called a regeneration of the NAC, occurs while the engine runs rich (resulting in excess hydrocarbons in the exhaust gas passing through the NAC). As the unused hydrocarbons pass over the trapped NOx, the NOx will join with the hydrocarbons to produce less harmful emissions, such as H2O and N2.
As a NAC ages over time, or when a NAC is defective, the catalytic sites on the catalyst bed for adsorbing NOx and oxygen become deactivated, which results in the NAC being less effective at trapping NOx and oxygen. Further, excess NOx emissions may occur if a NAC is inadvertently missing from the vehicle. Accordingly, current on-board diagnostic regulations require detection and communication to a user of ill-performing, degraded, or missing NAC to limit the emission of excess NOx into the atmosphere.
Conventional systems determine the performance and degradation of a NAC in a variety of ways. According to one system, the performance and degradation of the NAC is determined by attempting to estimate a lean-to-rich exhaust transition area subsequent rich-to-lean exhaust transition area. The estimated transition areas are based on a calculated difference between an NAC upstream air-fuel ratio value and a NAC downstream air-fuel ratio value, as well as an indication whether a regeneration event is occurring. In other systems, only those NAC input and output air-fuel ratio values obtained over a short period of time at the beginning of NAC regeneration events are used to determine the performance and degradation of the NAC.
According to conventional gasoline-powered engines using three-way catalysts (TWC), the performance of the TWC is determined by estimating the oxygen storage capacity of the TWC. More specifically, the performance of the TWC is determined by detecting changes in the oxygen storage capacity as the TWC ages.